Leeroy Jenkins

"Leeroy Jenkins" refers to a video capture of a World of Warcraft session widely circulated on the internet.

The clip is viewable here

In the clip, "a group of players discuss a detailed battle strategy while one of their number, Leeroy, is away from his computer. Their plan is ruined when Leeroy returns and, ignorant of the strategy, immediately charges headlong into battle shouting his own name as a battle cry. His companions rush to stop him, and his subsequent actions ruin the meticulous plan and the encounter ends in failure for the group. His response to the other players' reactions, "at least I have chicken", was also much parodied." (from Wikipedia)

Usage

To "pull a Leeroy," or to "Leeroy" is to engage in a rash, autonomous action that thwarts the well-laid plans of one's group and exposes the group to grave danger.